In recent years ruthenium oxide and its hydrate has been found to be excellent materials for high energy density electrodes because of their high capacitance and low resistance. See, for example, P. Kurzweil and O. Schmid, "High Performance Metal Oxide Supercapacitors," Proc. 6th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla. (1966). The outstanding capacitance properties of ruthenium oxide are postulated to arise in part from multiple oxidation states of ruthenium and the facile conversion of ruthenium from one oxidation state to another, and in part to proton mobility between the oxide and hydroxyl sites in hydrated ruthenium oxide. However, the observed capacitance of ruthenium oxide is quite dependent on its physical state. In particular, it has been recognized that high capacitance requires that ruthenium oxide be in the amorphous state with a high degree of hydration.
Energy storage in hydrated ruthenium oxide is associated with protons reacting with oxide species at the surface, consequently the surface area of the oxide determines its storage capability. It is clear that high surface area material is required for high energy storage density. Energy storage also is associated with the amorphous phase of the oxide, hence it also follows that a high energy density is favored by low crystallinity of ruthenium oxide. Jow et al. in U.S. Pat. No. 5,600,535 recognized that thin films of amorphous ruthenium oxide were highly desirable for electrode materials, and also recognized that electrodes require good adhesion of such films in order to provide a stable electrode. His solution to the problem of preparing the aforementioned materials was to prepare amorphous film electrodes by a sol-gel process where the ruthenium oxide precursor was coated on a substrate and subsequently annealed at temperatures which minimized the formation of any crystalline phase of ruthenium oxide. The patentees utilized as a substrate and current collector metals, especially titanium, although they mention that carbon powder also can be coated by their method. A specific capacitance as high as 430 farads/g was obtained by their process.
Jow formed his films wherein a sol-gel process was used to form a ruthenium alkoxide in non-aqueous solvents. A thin film of ruthenium alkoxide was deposited on titanium metal by, for example, dip coating, and the film thereafter was carefully annealed at 100-450.degree. C., a temperature range which minimized crystalline phase formation. The process was repeated to form a film of desired extent and thickness. However, the process was subject to several critical constraints. The patentees noted that a pH of 3-6 for RuCl.sub.3.xH.sub.2 O solution used as the ruthenium source was necessary for acceptable performance. The patentees also noted that the success of their method was quite dependent on the amount of alkali metal alkoxide used to form the ruthenium alkoxide; too much causes precipitation of ruthenium species and too little leads to poor adhesion of the subsequently formed film. Additionally, post treatment of coated metal is imperative to remove alkali metal from the film. The patentees also noted that the specific capacitance of the final material was quite dependent on the alkoxide used, even among such related alkoxides as ruthenium methoxide, ruthenium ethoxide, and ruthenium isopropoxide. Overall the process is time consuming with much opportunity of wasting expensive ruthenium oxide powder during repeated washing and drying required during processing.
To prepare a high energy storage density ruthenium oxide electrode it was imperative to develop an alternative to Jow's method. The process ought to be simple, with few if any critical variables. The process also ought to be efficient in ruthenium utilization, with few opportunities for ruthenium loss. The process in addition ought to be reliable, giving high performance material under a reasonable range of conditions. We have developed such a process as described and claimed below. Briefly, our process impregnates high surface area carbon fibers immobilized and dispersed in a cellulose matrix with an aqueous solution of a ruthenium source and subsequently converts the ruthenium deposited in the carbon fiber pores to highly disordered amorphous ruthenium oxide by heating the treated fiber in a controlled water atmosphere. Our process is simple and easy to carry out even by relatively unskilled operators, with virtually 100% utilization of ruthenium. The resulting ruthenium oxide-carbon fiber dispersed in a cellulose matrix shows an energy storage density, on a ruthenium oxide basis, close to 130 Joules/g, which is roughly equivalent to a specific capacitance of about 1040 farads/g ruthenium oxide.